A telephone network is often used as an interface between a digital modem and an analog modem. Generally, a digital modem is a device that communicates digital data by using digital signals that replicate analog waveforms. An analog modem is a device that communicates digital data by encoding the data on analog waveforms.
FIG. 1 shows a typical telephone network 99 for interconnecting a digital modem 101 and an analog modem 102. The digital modem 101 is usually interconnected with a digital network 113 via digital connections 112a, 112b. For instance, the digital modem 101 may be interconnected to a digital network 113 in the form of a public switch telephone network (PSTN) via a Local Exchange Carrier (LEC) subscriber loop. The digital network 113 may comprise, among other things, a T1 carrier system, a basic rate or primary rate Integrated Services Digital Network (ISDN), a fiber optic cable network, a coaxial cable network, a satellite network, or even a wireless digital communications network. Communications over the digital network 113 are conducted in accordance with a pulse code modulation (PCM) scheme. Channel capacity through these digital facilities is typically between 56 and 64 kilobits per second (kb/s). Coding of the signals is also employed so that compression and a constant signal/distortion performance over a wide dynamic range is achieved for optimal transmission of voice signals. A commonly used coding technique is a nonlinear mu-law coding.
The digital network 113 is in turn interconnected with another LEC subscriber loop that includes a coder/decoder (codec) 106. The codec 106 is interconnected with the digital network 113 via digital connections 114a, 114b. The codec 106 is often situated at a telephone company office or along a street near the analog modem subscriber in an SLC device. The codec 106 provides an interface between the digital network 113 and an analog telephone connection 118, sometimes referred to as a copper loop. For communications in the direction from the digital network 113 to the analog modem 102, the codec 106 includes a mu-to-linear-analog converter 109, which includes digital-to-analog (DAC) conversion functionality. The converter 109 converts nonlinear mu-law levels to a linear analog signal. For communications in the direction from the analog modem 102 to the digital network 113, the codec 106 includes a linear-analog-to-mu converter 107, which includes analog-to-digital (ADC) conversion functionality. The converter 107 converts the linear analog signal to nonlinear mu-law levels.
A hybrid 103 is in communication with the DAC and ADC via respective LPFs 111, 105. The hybrid 103 serves to separate the bidirectional analog signals from the analog telephone connection 118 into unidirectional transmit and receive analog signals sent to and received from the ADC 107 and the DAC 109, respectively.
Furthermore, the analog modem 102 is connected to the analog telephone connection 118 and communicates analog signals therewith. Thus, communications occur between the digital modem 101 and the analog modem 102 by way of the digital network 113 and the codec 106.
Researchers have been attempting to increase the speed at which data can be transferred through the telephone network between the digital and analog modems 101, 102. U.S. Pat. No. 5,394,437 to E. Ayanoglu et al. describes a high speed analog modem 102 that is synchronized to the DAC and ADC clocks of the codec 106. Further, a pulse level modulation scheme is utilized to communicate data along the telephone connection 118. With pulse level modulation, a plurality of voltage levels are communicated along the analog telephone connection 118. This system permits data transfer rates above 40 kb/s.
Although the aforementioned system is meritorious to an extent in terms of increasing data transfer rates, it suffers from various undesirable problems and disadvantages.
A primary disadvantage of the Ayanoglu system involves echo problems. Generally, there is sensitivity to quantized echoes because detection occurs at the codec quantizer, and there is an inability to provide echo cancellation prior to detection. More specifically, echo cancellation at the analog modem 102 is not a problem given its exceptional linearity. However, the echo at the codec is a major problem due to the mu-law coding and limited hybrid quality. On a poor subscriber loop, the receive signal is attenuated. The echo is increased due to the impedance mismatch. In fact, the echo level can exceed the receive signal level. Accordingly, both the analog modem 102 and the digital modem 101 will attempt to utilize all PCM levels. When the digital modem 101 echo results from one of the upper compander levels and the analog modem 102 has transmitted on one of the lower levels, then the echo will control the channel bank encoder step size. In this case, it is difficult to resolve the symbols from the analog modem 102.
Another disadvantage of the Ayanoglu system is that it requires complex timing synchronization with the codec.
Hence, there exists a need in the industry for systems and methods for increasing the speed of data transfers through a telephone network 99, which comprises both a digital and analog communications mediums, between a digital modem 101 and an analog modem 102.